The invention relates generally to apparatus for reading and processing signals from a magnetic recording medium and, more particularly, to an improved read circuit which avoids problems associated with differentiated droop and peak shift.
In contemporary data processing systems data is stored on magnetic tape or magnetic discs for retrieval and use at a later time. The data is recorded on one or more tracks of the magnetic medium in the form of a sequence of ones and zeros according to any one of several commonly used codes, such as the frequency modulation (FM) code, the modified frequency modulation (MFM) code, and the modified MFM (M.sup.2 FM) code. The last mentioned code has been found to be useful at high packing densities, where the data bits are closely compacted, in order to avoid peak shift problems.
Peak shift is a phenomenon which occurs primarily at high recording densities, since under these conditions the read head is also detecting the magnetic transitions immediately preceding and following the particular magnetic transitions which it is currently reading. If one of the adjacent transitions is further away from the one currently being read, the amplitude of the detected signal will not be reduced as much as by the more immediately adjacent transition, and this asymmetry causes the peak of the detection signal corresponding to the transition being read to be shifted from its true position. The resulting peak shift has a deleterious effect upon the detection circuitry of the magnetic playback apparatus, since clock bits may be confused with data bits and the timing circuitry of the magnetic recording apparatus may be thrown out of synchronization.
As mentioned above, the M.sup.2 FM code minimizes the problems associated with peak shift. However, it also introduces a problem of its own by causing the detection of false peaks or "shoulders". The nature of the alternating current signal detected by a read head transducing information bits in the M.sup.2 FM code is such that a "shoulder" frequently occurs, which "shoulder" does not represent a true peak. When the playback signal is differentiated, the shoulder may be interpreted as a peak, and when the differentiated signal is further processed in the read circuit, the output of the read circuit may indicate a false bit. It is important, of course, that the read circuit be designed so as to differentiate between true bits and false bits to preserve the integrity of the recorded information.
A known prior art magnetic read circuit is described in U.S. Pat. No. 4,012,785, in which circuitry is provided for detecting and eliminating false peaks from the playback signal. However, the read circuitry described in the aforementioned patent separately and independently processes the positive and negative peaks in the playback signal. This increases the likelihood of asymmetry in the read back system, since a greater or lesser amount of delay may be introduced in one branch of the read back circuit than in the other. Consequently, the prior art read back circuit may reintroduce undesirable peak shift into the playback signal, offsetting the advantages of the M.sup.2 FM recording code.